The present invention relates to heat dissipating devices, particularly for electronic semiconductor elements, and to methods of producing such devices.
Numerous conventional electronic semiconductor components, for example, simple components such as integrated circuits or also possibly silicon sensors, are applied, for example, as shown in FIG. 1, in the form of a semiconductor chip 1 to a housing base or bottom 4 via an adhesive or solder layer 6. On the one hand, the housing or, more precisely, the housing base 4, is intended to protect the chip against environmental influences. On the other hand, the housing base 4 should permit, via contact pins 3 provided therein and wires 2 bonded to the contact pins 3, the establishment of a releasable contact between the component and a base or printed circuit board 8. Moreover, the heat from the power consumed in the chip 1 is to be transferred by way of the housing bottom 4 and the contact pins 3 into the printed circuit board 8 and a heat dissipating body 9. In order to determine the temperature distribution between the chip 1 and the housing, heat balance considerations are made.
The heat generated in the silicon chip 1 is essentially transported from the chip to the housing by way of heat conduction (primarily heat diffusion). In the housing, the heat spreads out over the housing bottom 4 and heats it somewhat. This heat is removed, on the one hand, by thermal conduction from housing bottom 4 to housing cover 7 and, on the other hand, by way of convection into the air. Moreover, contact pins 3, in particular, remove heat from the housing bottom into the printed circuit board 8 disposed therebelow or into hybrid electronic ceramic devices. These considerations apply for standard housings such as, for example, TO5 and SOT housings, and similarly for special housings.
In SMD housings, heat is dissipated analogously from the heat generating chips into the encapsulation and to the legs of the housing. The silicon chips are here usually glued to bases. The electrical contacts between the silicon chips and the metal pins are produced by bonding.
The thermal energy to be dissipated generally develops in the circuits that are integrated in the surface of the silicon semiconductor chips. The heat generated by the power consumed there must be conducted initially through the silicon or other semiconductor material over a path of generally between 100 .mu.m and 600 .mu.m. This causes the thermal front to be broadened somewhat. The adhesive or solder then conducts the heat further into the metal base of the housing. A heat transfer resistance of about 0.42 W/K can here be observed in the adhesive. Conventional fastening techniques permit only a limited amount of heat to be conducted away from the silicon chip because the temperature of the chip surface sets itself as the equilibrium between the generated thermal energy and the dissipated thermal energy. FIG. 2 shows the resulting temperature curve for the various regions and transition locations of the semiconductor arrangement. The temperature of the chip surface, which for silicon may lie between a maximum of 125.degree. C. and 150.degree. C. is marked with the reference numeral 1. It is followed by the adhesive or solder location 6, the housing bottom 4, the metal/glass connection 10 between housing bottom 4 and printed circuit board or base 8, which is distinguished by contact heat conduction, the heat dissipating body 9, for example, in the form of a metal cooling sheet, and the environment, indicated by reference numeral 11, which is at room temperature.
The sketched temperature curve is the result of an exchange of heat that develops according to the above-mentioned equilibrium. The heat exchange must ensure that the maximum permissible temperature of the silicon between 125.degree. C. and 150.degree. C. is not exceeded, since at higher temperatures silicon becomes inherently conductive and the intended function of the electronic components can no longer be maintained. It must also be noted that already at temperatures between 100.degree. C. and 125.degree. C., the blocking voltages and the blocking currents of the diodes and transistors as well as other parameters become worse. It has been found that, with air cooling, it is possible to dissipate power losses of at most a few watts. Even with forced-air cooling, up to about 5 watts can be dissipated. This is also the case if, instead of an adhesive connection, so-called eutectic die bonds are employed which produce a lower heat transfer resistance of 0.33 W/K.
In the past, a number of passive cooling measures have been proposed which attempt to produce sufficient cooling during operation of the semiconductor simply by the conduction of heat with the aid of cooling bodies. For example, DE 3,735,818 Al discloses a passive cooling device in the form of an auxiliary silicon chip or substrate which simultaneously serves as a cooling body and a protective resistor for a light emitting element in chip form disposed on its top face. The rear surface of this auxiliary chip or substrate is fastened to a larger conventional cooling body which, in turn, is attached to a frame structure. However, such measures do not constitute a satisfactory solution of the described problems.
In order to improve heat dissipation, the substrate or printed circuit board, for example, is cooled. However, this does not provide for direct cooling of the chip without heat transfer resistances and is effective only in conjunction with intermediate layers that are good thermal conductors. Moreover, the great thermal capacity of such systems prevents the dissipation of, in particular, short heat pulses occurring in rapid succession without integratingly heating the cooling system. The possibility of a more direct and thus faster heat dissipation is discussed in the chapter, entitled "Verbesserung der Warmeableitung durch mikrostrukturiertes Silizium" (Improvement of Heat Dissipation Through Microstructured Silicon) of the book by A. Heuberger, entitled "Micromechanik" (Micromechanics), published by Springer Verlag, 1989, pages 480-483. It is proposed there to work troughs into the silicon and, in order to increase thermal conduction, fill them with metals such as gold, copper or silver. Heat transfer is then established in the chip to these troughs filled with little metal balls via metal connecting pins.
Even more effective cooling results if channels are formed in the underside of the silicon chip and a coolant, e.g., water, is actively conducted through these channels. The relatively deep troughs or channels required for this purpose may be formed either by sawing or by a special crystal orientation dependent etching technique as disclosed in chapter 3.2.1 "Anisotrope Atzverfahren" (Anisotropic Etching Methods), pages 128-140 of the above mentioned book. This technique is a wet chemical depth etching technique for micromechanical components employing various possible etching solutions. Troughs or grooves of a depth and width in a range from 0.1 .mu.m up to several 10 .mu.m can be easily realized with this technique.
The described liquid cooling results in very good heat dissipation, depending on the pumping speed up to a heat dissipating output of theoretically more than 1000 watts for a chip. However, short-term heat pulses in particular can be dissipated at sufficient speed only if flow rates are correspondingly high.
Additionally, IBM Technical Disclosure Bulletin, 1983, Volume 25, No. 8, pages 4118-4119 discloses a device in which channels are formed in the top face of a cooling chip by an anisotropic etching technique, with the cooling chip being bonded to the rear face of a smaller semiconductor chip. This arrangement is disposed in a so-called heat pipe structure which ensures that the liquid evaporated as a result of heat development in the semiconductor chip is transported to the cooler end of the heat pipe structure where the vapor condenses. The condensate is cooled by means of a heat sink and is then transported back into the channels by means of capillary effect. The channels in the cooling chip are intended to increase the surface area of the chip and serve to provide improved heat transfer.
The drawbacks of this closed device, which is based essentially on a mechanism for transporting gases or liquids to remove the heat, are obvious. Compared to the actual semiconductor chip, the entire arrangement is very large and operates only if the repeated or continuous filling of the channels by capillary action can be ensured for a long period of time. Also, the vapor atmosphere in the vicinity of an electronic unit is anything but ideal.
EP 0,251,836 Al, corresponding to U.S. Pat. No. 4,833,567, in principle, discloses a similar heat pipe structure with channels, that are partially filled with the evaporated liquid, being formed either directly within the chip or in a cap in the form of a chip of the same material that is glued to the chip. Again, a heat sink and a transporting device in the form of a wick are provided to conduct the heat away and to return the condensate. The heat is dissipated essentially by the described circulation mechanism for transporting the vapor to the cooling body and the return of the condensate.
According to DE-AS 2,441,613, a dielectric heat conducting layer of SiO.sub.2 having an upper and a lower cover layer of Si.sub.3 N.sub.4 is provided on the semiconductor chip, with inwardly widened channels being etched into the cover layer facing away from the semiconductor chip. Here again, the evaporation of a liquid absorbed into the heat conducting layer is utilized, and interruptions in the heat conducting layer are provided to ensure unimpeded evaporation. In order to realize the necessary condensate return, a wick structure is again provided within a housing.